libtracker-client: mark graph as nullable in vapi
libtracker-client: mark graph as nullable in vapi

This commit fixes the prototypes of the SparqlBuilder functions
insert_open and delete_open, marking the graph parameter as nullable.

outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker